This invention relates to insulation as applied to the inside of the exterior walls of transport aircraft. More particularly, this invention relates to a system for applying spray-on foam mixtures to the interior side of the external skin of the aircraft whereagainst the foam solidifies in place. Still more particularly, the invention includes draping a flexible barrier material between the sprayed-on foam and the external skin and structural and other members that protrude inwardly therefrom prior to application of the foam, which foam then completely covers all surfaces of such skin and members. Even more particularly, the invention relates to forming a semi-permeable layer on the outside of the foam to exclude liquid water therefrom.
Modern aircraft experience large temperature differentials between the inside and the outside of the fuselage that require the employment of insulation to moderate the interior temperature of the aircraft. A difficult challenge is created by exterior temperatures that are below those desired within the fuselage. A commercial airliner at cruising altitude experiences exterior temperatures significantly below freezing, often as low as -40.degree. C. The passengers and crew of the aircraft produce significant levels of humidity within the fuselage. This water vapor tends to condense on cold surfaces presented either on the interior cabin wall, within the insulation in the space between the interior cabin wall and the exterior skin of the aircraft, or on the inside of the exterior skin.
Condensation on the interior cabin wall is undesirable from a passenger comfort aspect. Condensation within the insulation is undesirable because it decreases the efficiency of the insulation and because the added weight from the condensed water increases the cost of operation of the aircraft and decreases its payload capacity. Condensation on the inside of the exterior skin can result in corrosion of the skin and the various structural members attached thereto resulting in decreased lifetime for the various components and the need to check for and repair or replace the corroded components. Corrosion on the inside of the exterior skin produced by condensation of this type and by other causes requires that these areas be able to be inspected at regular intervals. These inspections in turn require that the insulation system be readily removable in order that the inside of the exterior skin as well as the various attached structural members can be viewed and, as necessary, replaced or repaired.
The condensation problem could be largely eliminated if an optimized insulation system could be used. The most common insulation system in use today includes fiberglass bats or blankets that are enclosed by water-impermeable membranes, typically mylar, which are in turn perforated with small holes to prevent ballooning problems within the walls with the inevitable changes in pressure that occur as the airplane ascends and descends from altitude. These fiberglass blankets must be custom made for each aircraft type and, for each type, many individually different patterns must be used in order to effectively fill the open spaces between the interior cabin wall and the exterior skin while avoiding interference with the various structural and other assemblies also inside this space. Hence, these thermal blankets are expensive and difficult to install and maintain.
The thermal blankets work well when initially installed. However, the necessary perforations in the exterior membrane allow water vapor to enter into the blankets. It is estimated that this water vapor can increase the weight of the thermal blanket three fold within a three year period of typical airline service. For example, the thermal blankets in a twin jet aircraft, such as those in the MD-80 series, have been documented to retain, on average, between 330 and 1500 pounds of water. The DC-10/MD-11 trijet aircraft have been documented to retain, on average, between 660 and 2400 pounds of water. Because the wet insulation has a higher thermal conductivity than dry insulation, the heat transfer rate increases, thereby reducing the insulating effect of the blanket. It has been estimated that this added water weight increases the amount of fuel necessary to operate a twin jet aircraft by about $14,000 per year and that this absorbed water accounts for about 5% of the overall corrosion repair costs for the airplane.
The industry has searched for alternative insulation systems to overcome the above-described drawbacks and shortcomings of the conventional thermal insulation blankets. Many have turned to various types of foam systems. One such solution is proposed in U.S. Pat. No. 4,235,398 to William R. Johnson for "Thermal Insulation for Aircraft Fuselage." This system employs a variety of preformed rigid foam panels to provide the necessary thermal and noise insulation for an airliner. The foam used therein is a self-skimming type that minimizes the intrusion of water into the foam. The pre-formed panels have molded-in stand offs such that the foam panels only contact the exterior skin in the small areas of the stand offs, thereby allowing any water that condenses on the inside of the exterior skin to drain off to a central collection point. Although an improvement in certain respects over the fiberglass thermal blankets, this system still requires a large number of different shapes and sizes of preformed foam panels. Since each requires its own separate mold, the cost of this system is quite high.
Another solution is proposed in U.S. Pat. No. 5,611,504 to Haynes et al. for a "Semi-Rigid, Light Weight Fiber Glass/Polyimide Foam Sandwich Blanket Insulation." This blanket insulation system constitutes an evolutionary advancement over conventional fiberglass blankets, and includes fiberglass layers which are alternated with semi rigid layers of polyimide foam. The polyimide foam adds structural rigidity to the fiber glass bats, enhances the sound absorption, and allows for easier installation. However, it does not significantly mitigate the water absorption problem, and it is similar to the conventional thermal blankets with respect to method and cost of fabrication.
Also of interest is U.S. Pat. No. 5,251,849 to Milton J. Torres for "Strain Reduced Airplane Skin." Although not directed toward providing insulation for an aircraft fuselage, this reference teaches the use of a formed in place polyisocyanuarte solid closed cell foam material to increase the strength of the airplane structure. Because of their low density, lack of flammability, relative imperviousness to water, and strength, other foams such as polyurethane or silicone can be used. Since the foam produces a rigid, structural component, it cannot be removed once it is formed in place. However, this reference does not even mention, much less address the absolute requirement of being able to visibly inspect the inside surface of the exterior skin and the various attached structural elements, such as frames and stringers. Employment of a structural foam in the manner taught in this reference precludes the possibility of making these mandatory, visible corrosion inspections. In contrast, a primary advantage of the system of the present invention is that the foam used is somewhat flexible and can be easily removed for inspections.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a system and method capable of providing an effective and low cost insulation system for an aircraft fuselage, which overcomes the above-described drawbacks and shortcomings of the presently available technology. The present invention fulfills these needs in the art.